The present invention provides a novel peptide that is derived from a loop or xe2x80x9clollipopxe2x80x9d region of transforming growth factor alpha (TGF-xcex1) and is biologically active for causing stem cells to proliferate and migrate. The present invention further provides a method for augmenting hematopoiesis, particularly trilineage hematopoiesis, and a method for suppressing immune functioning associated with autoimmune diseases, and a method for suppressing inflammatory responses mediated (in part) by excessive histamine release, comprising administering an effective amount of a TGF-xcex1 polypeptide or a fragment thereof, such as the lollipop region. The present invention further provides a method for treating or preventing mucositis and gastrointestinal side effects in patients undergoing cancer treatment, comprising administering an effective amount of a TGF-xcex1 polypeptide or a fragment thereof, such as the lollipop region.
There are several disease treatments that could significantly benefit by having cells regenerate after injury or lesion formation, particularly in the Central Nervous System (CNS), in the immune system and in the gastrointestinal tract. The expression of growth factors and their receptors on the preimplanted human embryo and maternal reproductive tract indicates that such factors influence growth and differentiation of embryonic cells in an autocrine and paracrine manner. Such growth factors are peptides that variously support survival, proliferation, differentiation, size and function of nerve cells and other lineages of ells. EGF (epidermal growth factor) is the first member found of the EGF family and characterized many years ago (Savage and Cohen, J. Biol. Chem. 247:7609-7611, 1972; and Savage et al.; J. Biol. Chem. 247:7612-7621, 1972). Additional members of the EGF family have been found and they include vaccina virus growth factor (VGF; Ventatesan et al., J. Virol. 44:637-646, 1982); myxomavirus growth factor (MGF; Upton et al. J Virol. 61:1271-1275, 1987), Shope fibroma virus growth factor (SFGF; Chang et al., Mol. Cell. Biol. 7:535-540, 1987), amphiregulin (AR; Kimura et al., Nature 348:257-260, 1990), and heparin binding EGF-like factor (HB-EGF; Higashiyama et al., Science 251:936-939, 1991). A common structural feature of these polypeptides is the presence of six cysteine residues that form three disulfide cross links that support a conserved structure that binds to the EGF receptor.
Another member of the EGF family is TGFxcex1 and it also binds to the EGF receptor (Todaro et al., Proc. Natl. Acad. Sci. USA 77:5258-5262, 1980). TGFxcex1 stimulates the EGF receptor""s tyrosine kinase activity and has many cellular functions, such as stimulating a mitogenic response in a wide variety of cell types. TGFxcex1 and EGF mRNAs reach their highest levels and relative abundance (compared to total RNA in the early postnatal period and decrease thereafter, suggesting a role in embryonic development. From a histological perspective, TGFxcex1 acts on numerous cell types throughout the body. The active form of TGFxcex1 is derived from a larger precursor and contains 50 amino acids. TGFxcex1 shares only a 30% structural similarity with the 53-amino acid form of EGF, but including conservation of all six cysteine residues. TGFxcex1 is highly conserved among species. For example, the rat and human polypeptides share about 90% homology as compared to a 70% homology as between the rat and human EGF polypeptide. The amino acid sequence of human TGFxcex1 is shown in SEQ ID NO. 1. The sequence shows that a family consisting of vaccinia growth factor, amphiregulin precursor, betacellulin precursor, heparin binding EGF-like growth factor, epiregulin (rodent only), HUS 19878 and schwannoma derived growth factor have similar sequence motifs and can be considered as members of the same family based upon their shared cysteine disulfide bond structures.
TGFxcex1 is an acid and heat stable polypeptide of about 5.6 kDa molecular weight. It is synthesized as a larger 30-35 kDa molecular weight glycosylated and membrane-bound precursor protein wherein the soluble 5.6 kDa active form is released following specific cleavage by an elastase-like protease. TGFxcex1 binds with high affinity in the nanomolar range and induces autophosphorylation to transduce signal with the EGF receptor. TGFxcex1 is 50 amino acids in length and has three disulfide bonds to forms its tertiary configuration. All three disulfide bonds must be present for activity. TGFxcex1 is stored in precursor form in alpha granules of secretory cells. Moreover, the primary amino acid sequence is highly conserved among various species examined, such as more than 92% homology at the amino acid level as between human and rat TGFxcex1 polypeptides.
TGFxcex1 has been investigated extensively and has recently been identified as useful for treating a patient with a neurological deficit. This mechanism is thought to stimulate proliferation and migration of neural-origin stem cells to those sites or lesions in a deficit. For example, Parkinson""s Disease is characterized by resting tremor, rigidity, inability to initiate movement (akinesia) and slowness of movement (bradykinesia). The motor deficits are associated with progressive degeneration of the dopaminergic innervation to the nucleus accumbens and degeneration of noradrenergic cells of the locus ceruleus and serotonergic neurons of the raphe. Up to 80% of nigral dopamine neurons can be lost before significant motor deficits are manifest. TGFxcex1 (full polypeptide) was shown, when infused into rat brains, was useful for the treatment of neurodegenerative disorders. Intracerebroventricular (ICV) or intrastriatal infusions of TGFxcex1 induced neuronal stem cell proliferation, but degenerating or damaged or otherwise abnormal cells needed to be present to facilitate migration of the neuronal stem cells to a site of injury on a scale sufficient to impact recovery from an associated neurological deficit. Forebrain neural stem cells, that give rise to migrating progenitor cells that affect treatment and recovery from a neurological deficit disorder, are the migrating cells that affect treatment recovery from a neural deficit disorder (e.g., Parkinson""s Disease, Huntington""s Disease, Alzheimer""s Disease and the like).
Neural stem cells have been found in subependyma throughout the adult rodent CNS (Ray et al. Soc. Neurosci. 22:394.5, 1996) and in the subependyma of adult human forebrain (Kirschenbaum et al., Cerebral Cortex 4:576-589, 1994). Thus, the discovery that TGFxcex1 stimulates proliferation of neural stem cells and promotes migration to a site of injury or deficit has led to its investigation for the treatment of a neurodegenerative disorder (Alzheimer""s Disease, Huntington""s Disease and Parkinson""s Disease) or CNS traumatic injury (e.g., spinal chord injury), demyelinating disease, CNS inflammatory disease, CNS autoimmune disease (e.g., multiple sclerosis) or CNS ischemic disease (e.g., stroke or brain attack).
A CNS stem cell has the potential to differentiate into neurons, astrocytes and to exhibit replication of itself to provide a resource for self-renewal. Both neurons and glial cells seem to be derived from a common fetal precursor cell. In the vertebrate CNS, multipotential cells have been identified in vitro and in vivo. Certain mitogens, such as TGFxcex1, can cause proliferation of CNS mutipotential cells in vitro and this is the basis for a procedure to harvest such cell, treat them ex vivo to stimulate proliferation in culture and then readminister such cells. Immunohistochemical analysis in the human brain supports the notion that TGFxcex1 is widely distributed in neurons and glial cells both during development and during adulthood. In mice genetically altered to lack expression of functioning TGFxcex1, there was a decrease in neural progenitor cell proliferation in forebrain subependyma, providing evidence for TGFxcex1 as a proliferative factor for neural progenitor cells.
TGFxcex1 is found mainly in various neurons of the CNS during development and in the adult brain in the cerebral neocortex, hippocampus and striatum. It is also found in glial cells, primarily in the cerebral and cerebellar cortex areas. Northern blot analyses showed that TGFxcex1 and not EGF (epidermal growth factor) is the most abundant ligand that binds to the EGF receptor in the brain. TGFxcex1 mRNA levels were 15-170 times higher than EGF in cerebellum and cerebral cortex. TGFxcex1 also appears in germinal centers of the brain during neurogenesis and gliogenesis in the developing brain. In the midbrain, the distribution of TGFxcex1 overlaps with tyrosine hydroxylase mRNA and fetal dopaminergic neurons. In culture, TGFxcex1 enhanced survival and neurite outgrowth of neonatal rat dorsal ganglion neurons (EGF did not) and survival and differentiation of CNS neurons. TGFxcex1 induced proliferation of neural precursor cells of the murine embryonic mesencephalon and further induced a significant increase in the number of astroglia and microglia in fetal rat medial septal cells. TGFxcex1 increased glutamic acid decarboxylase activity and decreased choline actetyltransferase activity. Thus, TGFxcex1 acted as a general neuronal survival factor affecting both cholinergic and GABAergic neurons. In addition, TGFxcex1 is a mitogen for pluripotent brain stem cells. Forebrain subependyma contains nestin positive neural stem cells and their progeny, which are constitutively proliferating progenitor epithelial cells. A xe2x80x9cknockoutxe2x80x9d mouse that was genetically engineered to delete the gene for TGFxcex1 showed a reduction in neuronal progenitor cells in the subependyma and a reduction in neuronal progenitors that migrate to the olifactory bulb. In vitro, TGFxcex1 promoted dopamine uptake in fetal rat dopaminergic neurons in a dose-dependent and time-dependent manner. TGFxcex1 selectively promoted dopaminergic cell survival, enhanced neurite length, branch number and the soma area of tyrosine hydroxylase immunopositive cells. The levels of TGFxcex1 were elevated in ventricular cerebrospinal fluid in juvenile parkinsonism and Parkinson""s Disease and may represent a compensatory response to neurodegeneration. Further, TGFxcex1 prevented a striatal neuronal degeneration in an animal model of Huntington""s Disease.
The mucosal epithelium of the intestine is in a continually dynamic state known as xe2x80x9cepithelial renewalxe2x80x9d in which undifferentiated stem cells from a proliferative crypt zone divide, differentiate and migrate to the luminal surface. Once terminally differentiated, they are sloughed from the tips of the villi. The turnover of the crypt-villus cell population is rapid and occurs every 24-72 hours. Continuous exfoliation of the cells at the villus tip is counterbalanced by ongoing proliferation in the crypt so that net intestinal epithelial mass remains relatively constant. The rapidly-proliferating epithelium of the gastrointestinal tract is extremely sensitive to cytotoxic drugs that are widely used in the chemotherapy of cancer. This xe2x80x9cside effectxe2x80x9d reduces the tolerated dose of such drugs as it can cause a breakdown of the GI barrier function and septic condition in a patient already immuno-compromised. This can also lead to life-threatening hemorrhage. Therefore, there is a need in the art for the development of products and delivery systems that stimulate the repair and rejuvenation of mucosal epithelium in the gastrointestinal tract to provide benefit to patient receiving chemotherapy and radiation therapy for cancer.
Therefore, there is a need in the art to find improved TGFxcex1 mimetic agents that are more economical to produce and are smaller (in terms of molecular weight). The present invention was made to address such a need.
The present invention is based upon two basic discoveries that have not been reported before in the literature of TGFxcex1. Firstly, a novel genus of small peptides, much smaller than (50 amino acid human) TGFxcex1, was discovered as having TGFxcex1 biological activity and therefore are useful as pharmacologic (therapeutic) agents for the same indications as full-length TGFxcex1 polypeptide (50 or 57 amino acids). Secondly, TGFxcex1 and the genus of smaller peptide fragments disclosed herein, were found to have therapeutic activity to stimulate hematopoiesis in patients undergoing cytotoxic cancer chemotherapy and to act as a cytoprotective agent to protect a patient undergoing cancer cytotoxic therapy from gastrointestinal (GI) side effects, such as mucositis and otherwise support the barrier function of the GI tract when it is harmed by cytotoxic therapy.
The present invention provides a compound that acts as a TGF-xcex1 mimetic, comprising at least an 11-membered peptide compound from formula I:
-X1a-Cys-His-Ser-X1b-X2-X1a-X1b-X1a-X3-Cys (SEQ ID NO: 4)xe2x80x83xe2x80x83I
wherein X1a and X1b are independently Val, Gly or Ala wherein X2 is Tyr or Phe, wherein X3 is Arg or Lys, and wherein the two Cys moieties form a disulfide bond to create an 11-amino acid loop peptide. Preferably at least one or more of the following seven amino acids, are added to the C terminus Cys residue from formula II:
-X4-His-X1c-X4-X5-X6-X1c (SEQ ID NO: 5)xe2x80x83xe2x80x83II
wherein X4 is Glu or Asp, wherein X5 is Leu or Ile, and wherein X6 is Asp, Glu or Leu. Preferably, X1a is Val, X1b is Gly in formula I, and X1c is Ala in formula II. Preferably X2 is Tyr and X3 is Arg. Most preferably, the loop peptide is 13 amino acids in length wherein X1a is Val, X1b is Gly, X1c is Ala and X4 is Glu.
The present invention finer provides a pharmaceutical composition comprising a loop peptide in a pharmaceutically acceptable carrier, wherein the loop peptide compound comprises at least an 11-membered peptide compound from formula I:
-X1a-Cys-His-Ser-X1b-X2-X1a-X1b-X1a-X3-Cys (SEQ ID NO: 4)xe2x80x83xe2x80x83I
wherein X1a and X1b are independently Val, Gly or Ala wherein X2 is Tyr or Phe, wherein X3 is Arg or Lys, and wherein the two Cys moieties form a disulfide bond to create an 11-amino acid loop peptide. Preferably at least one or more of the following seven amino acids, are added to the C terminus Cys residue from formula II:
xe2x80x83-X4-His-X1c-X4-X5-X6-X1c (SEQ ID NO: 5)xe2x80x83xe2x80x83II
wherein X4 is Glu or Asp, wherein X5 is Leu or Ile, and wherein X6 is Asp, Glu or Leu. Preferably, X1a is Val, X1b is Gly in formula I, and X1c is Ala in formula II. Preferably X2 is Tyr and X3 is Arg. Most preferably, the loop peptide is 13 amino acids in length wherein X1a is Val, X1b is Gly, X1c is Ala and X4 is Glu.
The present invention further provides a method for treating a neurodegenerative disease with a pharmaceutically active loop peptide or a pharmaceutically active TGFxcex157 polypeptide, wherein the loop peptide comprises at least an 11-membered peptide compound from formula I or a polypeptide from formula III, wherein formula I is:
X1a-Cys-His-Ser-X1b-X2-X1c-X1b-X1a-X3-Cys (SEQ ID NO: 4)xe2x80x83xe2x80x83I
wherein X1a and X1b are independently Val, Gly or Ala wherein X2 is Tyr or Phe, wherein X3 is Arg or Lys, and wherein the two Cys moieties form a disulfide bond to create an 11-amino acid loop peptide; wherein formula III is:
Loop peptide-N-terminus-linker-cyclic C4H8N2-linker-Loop peptide-N-terminusxe2x80x83xe2x80x83II
wherein the linker moiety is designed to link the N-terminus of the Loop peptide to a nitrogen atom of the ring C4H8N2 and wherein the xe2x80x9cloop peptidexe2x80x9d comprises at least an 11-membered peptide compound from formula I; wherein X1a and X1b are independently Val, Gly or Ala wherein X2 is Tyr or Phe, wherein X3 is Arg or Lys, and wherein the two Cys moieties form a disulfide bond to create an 11-amino acid loop peptide; and
wherein TGFxcex157 is a 57 amino acid polypeptide having the formula IV:
Ser-Leu-Ser-Leu-Pro-Ala-Met-N-Human TGFxcex1-COOH (SEQ ID NO: 3)xe2x80x83xe2x80x83IV
wherein human TGFxcex1 is a 50 amino acid polypeptide having the formula of SEQ ID NO: 1. Preferably at least one or more of the following seven amino acids, are added to the C terminus Cys residue from formula II:
-X4-His-X1c-X4-X5-X6-X1c (SEQ ID NO: 5)xe2x80x83xe2x80x83II
wherein X4 is Glu or Asp, wherein X5 is Leu or Ile, and wherein X6 is Asp, Glu or Leu. Preferably, X1a is Val, X1b is Gly in formula I, and X1c is Ala in formula II. Preferably X2 is Tyr and X3 is Arg. Most preferably, the loop peptide is 13 amino acids in length wherein X1a is Val, X1b is Gly, X1c is Ala and X4 is Glu.
The present invention further provides a method for enhancing hermatopoiesis during cytotoxic or immune-suppressing therapy, comprising administering a TGFxcex1 polypeptide or a TGFxcex157 polypeptide or a pharmaceutically active loop peptide or a combination thereof, wherein the loop peptide comprises at least an 11-membered peptide compound from formula I:
-X1a-Cys-His-Ser-X1b-X2-X1a-X1b-X1c-X3-Cys (SEQ ID NO: 4)xe2x80x83xe2x80x83I
wherein X1a and X1b are independently Val, Gly or Ala wherein X2 is Tyr or Phe, wherein X3 is Arg or Lys, and wherein the two Cys moieties form a disulfide bond to create an 11-amino acid loop peptide;
and wherein TGFxcex157 is a 57 amino acid peptide having the formula IV:
Ser-Leu-Ser-Leu-Pro-Ala-Met-N-Human TGF xcex1-COOH (SEQ ID NO: 3)xe2x80x83xe2x80x83IV
wherein human TGFxcex1 is a 50 amino acid polypeptide having the formula of SEQ ID NO: 1. Preferably at least one or more of the following seven amino acids, are added to the C terminus Cys residue from formula II:
-X4-His-X1c-X4-X5-X6-X1c (SEQ ID NO: 5)xe2x80x83xe2x80x83II
wherein X4 is Glu or Asp, wherein X5 is Leu or Ile, and wherein X6 is Asp, Glu or Leu, Preferably, X1a is Val, X1b is Gly in formula I, and X1c is Ala in formula II. Preferably X2 is Tyr and X3 is Arg. Most preferably, the loop peptide is 13 amino acids in length wherein X1a is Val, X1b is Gly,3 X1c is Ala and X4 is Glu.
Preferably the invention further comprises administering a second hematopoietic growth factor agent to stimulate more mature hematopoietic precursor cells, wherein the second hematopoietic growth factor is selected from the group consisting of erythropoietin, thrombopoietin, G-CSF (granulocyte colony stimulating factor), and GM-CSF (granulocyte macrophage colony stimulating factor). Preferably, the invention further comprises administering stem cell factor (SCF) to augment CD34 positive progenitor cells.
The present invention further provides a method for treating or preventing mucositis of the gastrointestinal tract caused by cytotoxic or immune-suppressing therapy, comprising administering a TGFxcex1 polypeptide or a TGFxcex157 polypeptide or a pharmaceutically active loop peptide or a combination thereof, wherein the loop peptide comprises at least an 11-membered peptide compound from formula I:
-X1a-Cys-His-Ser-X1b-X2-X1a-X1b-X1c-X3-Cys (SEQ ID NO: 4)xe2x80x83xe2x80x83I
wherein X1a and X1b are independently Val, Gly or Ala wherein X2 is Tyr or Phe, wherein X3 is Arg or Lys, and wherein the two Cys moieties form a disulfide bond to create an 11-amino acid loop peptide;
and wherein TGFxcex157 is a 57 amino acid peptide having the formula IV:
Ser-Leu-Ser-Leu-Pro-Ala-Met-N-Human TGFxcex1-COOH (SEQ ID NO: 3)xe2x80x83xe2x80x83IV
wherein human TGFxcex1 is a 50 amino acid polypeptide having the formula of SEQ ID NO: 1. Preferably at least one or more of the following seven amino acids, are added to the C terminus Cys residue from formula II:
-X4-His-X1c-X4-X5-X6-X1c (SEQ ID NO: 5)xe2x80x83xe2x80x83II
wherein X4 is Glu or Asp, wherein X5 is Leu or Ile, and wherein X6 is Asp, Glu or Leu. Preferably, X1a is Val, X1b is Gly in formula I, and X1c is Ala in formula II. Preferably X2 is Tyr and X3 is Arg. Most preferably, the loop peptide is 13 amino acids in length wherein X1a is Val, X1b is Gly, X1c is Ala and X4 is Glu.
The present invention further provides a bifunctional compound that acts as a TGFxcex1 mimetic, comprising a compound from formula III:
Loop peptide-N-terminus-linker-cyclic C4H8N2-linker-Loop peptide-N-terminusxe2x80x83xe2x80x83III
wherein the linker moiety is desired to link the N-terminus of the Loop peptide to a nitrogen atom of the ring C4H8N2 and wherein the xe2x80x9cloop peptidexe2x80x9d comprises at least an 11-membered peptide compound from formula I:
-X1a-Cys-His-Ser-X1b-X2-X1a-X1b-X1c-X3-Cys (SEQ ID NO: 4)xe2x80x83xe2x80x83I
wherein X1a and X1b are independently Val, Gly or Ala wherein X2 is Tyr or Phe, wherein X3 is Arg or Lays, and wherein the two Cys moieties form a disulfide bond to create an 11-amino acid loop peptide; and wherein TGPxcex157 is a 57 amino acid polypeptide having the formula IV:
Ser-Leu-Ser-Leu-Pro-Ala-Met-N-Human TGFxcex1-COOH (SEQ ID NO: 3)xe2x80x83xe2x80x83IV
wherein human TGPxcex1 is a 50 amino acid polypeptide having the formula of SEQ ID NO: 1. Preferably at least one or more of the following seven amino acids, are added to the C terminus Cys residue from formula II:
-X4-His-X1c-X4-X5-X6-X1c (SEQ ID NO: 5)xe2x80x83xe2x80x83II
wherein X4 is Glu or Asp, wherein X5 is Leu or Ile, and wherein X6 is Asp, Glu or Leu. Preferably, X1a is Val, X1b is Gly in formula I, and X1c is Ala in formula II. Preferably X2 is Tyr and X3 is Arg. Most preferably, the loop peptide is 13 amino acids in length wherein X1a is Val, X1b is Gly, X1c is Ala and X4 is Glu.
The present invention further provides a method for treating inflammatory bowel disease, colitis, and Chron""s Disease of the gastrointestinal tract, comprising administering a TGFxcex1 polypeptide or a TGFxcex157 polypeptide or a pharmaceutically active loop peptide or a combination thereof, wherein the loop peptide comprises at least an 11-membered peptide compound from formula I:
-X1a-Cys-His-Ser-X1b-X2-X1a-X1b-X1a-X3-Cys (SEQ D NO: 4)xe2x80x83xe2x80x83I
wherein X1a and X1b are independently Val, Gly or Ala wherein X2 is Tyr or Phe, wherein X3 is Arg or Lys, and wherein the two Cys moieties form a disulfide bond to create an 11-amino acid loop peptide;
and wherein TGFxcex157 is a 57 amino acid peptide having the formula IV:
Ser-Leu-Ser-Leu-Pro-Ala-Met-N-Human TGFxcex1-COOH (SEQ ID NO: 3)xe2x80x83xe2x80x83IV
wherein human TGFxcex1 is a 50 amino acid polypeptide having the formula of SEQ ID NO: 1. Preferably at least one or more of the following seven amino acids, are added to the C terminus Cys residue from formula II:
-X4-His-X1c-X4-X5-X6-X1c (SEQ ID NO: 5)xe2x80x83xe2x80x83II
wherein X4 is Glu or Asp, wherein X5 is Leu or Ile, and wherein X6 is Asp, Glu or Leu. Preferably, X1a is Val, X1b is Gly in formula I, and X1c is Ala in formula II. Preferably X2 is Tyr and X3 is Arg. Most preferably, the loop peptide is 13 amino acids in length wherein X1a is Val, X1b is Gly, X1c is Ala and X4 is Glu.
The present invention further provides a method for treating an inflammatory reaction of autoimmune diseases, comprising administering a TGFxcex1 polypeptide or a TGFxcex157 polypeptide or a pharmaceutically active loop peptide or a combination thereof, wherein the loop peptide comprises at least an 11-membered peptide compound from formula I:
-X1a-Cys-His-Ser-X1b-X2-X1a-X1b-X1a-X3-Cys (SEQ ID NO: 4)xe2x80x83xe2x80x83I
wherein X1a and X1b are independently Val, Gly or Ala wherein X2 is Tyr or Phe, wherein X3 is Arg or Lys, and wherein the two Cys moieties form a disulfide bond to create an 11-amino acid loop peptide;
and wherein TGFxcex157 is a 57 amino acid peptide having the formula IV:
Ser-Leu-Ser-Leu-Pro-Ala-Met-N-Human TGFxcex1-COOH (SEQ ID NO: 3)xe2x80x83xe2x80x83IV
wherein human TGFxcex1 is a 50 amino acid polypeptide having the formula of SEQ ID NO; 1. Preferably the autoimmune diseases are selected from the group consisting of Type II (Juvenile) Diabetes, rheumatoid arthritis, lupus and multiple sclerosis. Preferably, at least one or more of the following seven amino acids, are added to the C terminus Cys residue from formula II:
-X4-His-X1c-X4-X5-X6-X1c (SEQ ID NO: 5)xe2x80x83xe2x80x83II
wherein X4 is Glu or Asp, wherein X5 is Leu or Ile, and wherein X6 is Asp, Glu or Leu. Preferably, X1a is Val, X1b is Gly in formula I, and X1c is Ala in formula II. Preferably X2 is Tyr and X3 is Arg. Most preferably, the loop peptide is 13 amino acids in length wherein X1a is Val, X1b is Gly, X1c is Ala and X4 is Glu.